It is highly unlikely that Lego would approve this. They just disapproved a Shawn Of The Dead set due to the fact that the subject matter of the movie is not appropriate for Lego's 6-10 year old target market. They would probably have the same issue with Serenity.

RayHill, Lego has done plenty of other sets for other action-adventure franchises like Pirates of the Caribbean and Batman, so it's not unthinkable that they'd approve it. I love Shawn of the Dead, but I can see how it might not have made the cut.